How It Works: Turbocharging

This component compresses air going into your engine for more power, but there is a fuel economy cost

This component compresses air going into your engine for more power, but there is a fuel economy costJennifer Fravica

It used to be that turbochargers were mostly used on high-performance sports cars. They still give go-fast cars an extra boost of power, but increasingly, automakers use them on smaller engines to boost power when needed but with better overall fuel economy. They’re also used on virtually all diesel engines to produce more power.

A turbocharger is basically an air pump, pushing extra oxygen into the engine as needed so it can burn more fuel to make more power.

Engines contain pistons, which move up and down in cylinders. These turn a heavy central crankshaft, the same way your legs move up and down to power a bicycle. The crankshaft’s spinning motion is used to turn the vehicle’s wheels.

An Audi 3.0-L V6 engine with two turbochargers arranged in series.Audi

What makes it all move is a vapour of air and gasoline at the top of the piston. When that’s ignited by the spark plug, the force of combustion pushes the piston down to turn the crank. The burned gases are then expelled as exhaust.

Each piston slides down at the start of its cycle, creating a vacuum. In a non-turbo engine, known as naturally-aspirated, air rushes in when the intake valve opens, but it can only fill the cylinder at atmospheric pressure. Burning more fuel makes more power, but since the fuel/air mixture must be precise for the engine to run properly, adding more gasoline won’t work, and the cylinder can’t pull in any extra air.

In a turbocharged engine, the turbo pumps in a higher volume of air under pressure, and the vehicle’s computer responds by adding the correct amount of additional fuel.

The turbo is powered by the exhaust gases. One side of the turbo is located at the exhaust manifold, the other at the engine’s air intake, and it contains two small fans joined by a shaft. As exhaust passes through the turbo, it spins one fan, called the turbine. This in turn spins the second fan, called the compressor, which draws in fresh air, pressurizes it, and forces it into the engine. The difference between atmospheric pressure and the amount of air pressure the turbo provides is known as boost, and is measured in pounds per square inch (psi).

Instead of a turbo, some vehicles use a supercharger, which also forces in air but runs mechanically off the engine’s crankshaft, instead of the exhaust stream.

A cutaway of a turbocharger, showing the turbine and compressor fans joined by a shaft.Audi

One of the issues with turbocharging is that air heats up as it’s compressed, and that’s the opposite of what you want. Cool air is more oxygen-dense, and so it can be mixed with more fuel and still combust properly in the cylinder. Automakers add a heat exchanger called an intercooler to the turbo system, which absorbs heat and reduces the temperature of the air entering the engine’s cylinders.

The turbo’s fans spin very fast – as much as 250,000 revolutions per minute or more – and there’s potential for too much pressure in the engine under maximum load. If this happens, a valve called a waste gate opens up, diverting some of the exhaust gases away from the turbine.

The turbocharger doesn’t boost the engine all the time. If you’re driving moderately, the air drawn in at atmospheric pressure is enough, and the engine operates like it’s naturally-aspirated. When you hit the throttle, the engine works harder and creates more exhaust pressure. This spins the turbocharger, which in turn boosts the engine, which in turn receives more fuel – which is why these small-displacement engines can suddenly become a lot thirstier than expected when you drive them hard. (On the plus side, that extra oxygen tends to burn the fuel in the cylinder more completely, increasing the engine’s efficiency and reducing harmful emissions.)

The turbocharger also creates a headache for engineers because it doesn’t immediately work at full capacity. There’s a short delay between the time you put your foot down and when the turbocharger spins to sufficient speed to provide boost and give you the desired burst of acceleration. This is known as turbo lag.

It used to be far more noticeable in older cars, but today, automakers use different methods to help reduce it. Lightweight turbine vanes are used, so it takes less pressure to spin them. Smaller turbochargers spool up faster and some automakers put two of them on an engine, combining a small one for quick initial boost with a larger one that can provide more power at higher engine speeds. A handful of automakers, including Volvo, use both a mechanically-driven supercharger and exhaust-driven turbocharger together on the engine to achieve this.

Another technology is variable geometry, which automatically adjusts the way the exhaust gas flows into the turbine wheel depending on engine speed and power requirements.

Turbocharged engines generally don’t require any additional maintenance, other than following the vehicle’s recommended oil changes and spark plug replacement. Some newer turbo engines run fine on regular-grade gasoline, but check your owner’s manual for any premium-grade requirement.

Most automakers just say “turbocharged,” but a few use proprietary names, such as Audi’s TFSI (for turbo fuel stratified injection) or Ford’s EcoBoost. If you’re not sure, ask if it’s a turbo before you buy.